This invention relates to a gas turbine engine rotor assembly and in particular to a method of manufacturing a gas turbine engine rotor assembly.
Rotor assemblies which are intended for use in gas turbine engines usually comprise a disc having an annular array of aerofoil blades equally spaced apart around its radially outer periphery. The aerofoil blades are usually attached to the disc by some form of mechanical connection. Thus for instance it is common to provide each aerofoil blade with a fir-tree configuration root which locates in a correspondingly shaped cut-out portion in the disc periphery. While such methods of attachment are satisfactory when employed in medium to large size gas turbine engines, they are not so successful when employed in small gas turbine engines.
Because of the physical limitations in manufacturing small scale components, rotor blade aerofoil cross sections are disproportionately large in comparison with the rotor disc to which they are required to be fixed. They therefore in turn require relatively large fir-tree type root fringes to adequately retain them on the disc at high rotational speeds.
As speeds and temperature are increased in order to meet the demands of impaired performance, then the proportions of fir-tree type fixings need to be increased still further. Thus a mechanical limitation is reached with overcrowding of the blade root fixings on the rotor disc periphery. It is therefore desirable to provide an alternative method of attaching rotor blades to discs in which the use of mechanical root fixings is eliminated, thereby alleviating the problem of overcrowding on the disc periphery.
In UK Patent No. 1586331 there is described a method of manufacturing a rotor assembly in which the disc is formed with a plane periphery and the aerofoil blades constitute part of a ring which is adapted to fit onto that periphery. The joint lines between the ring and the disc are brazed under vacuum after which the assembly is hot isostatically pressed to achieve diffusion bonding across the interface between the ring and disc. Thus the method avoids the overcrowding and consequent mechanical limitation of conventional root fixing on a small diameter disc since there is no mechanical connection between the aerofoil blades and the disc.
One advantage of producing rotor assemblies by this method is that of achieving satisfactory location of the ring on the disc. Thus since it is difficult to achieve abutting surfaces of the ring and disc which match exactly, there is a possibility that some areas will abut while others will be spaced apart by varying distances. This in turn can lead to problems in achieving satisfactory sealing of the joint lines betwen the ring and disc and also variability in the quality of the diffusion bond across the interface between the ring and disc.
British Patent Application Nos. 2106425A and 2106016A disclose gas turbine rotors which are made by diffusion bonding preformed aerofoil blades to the periphery of a disc by using a hot isostatic pressure technique. Initially the interface between the blades and the periphery of the disc is sealed by a brazing step. To prevent the braze material from contaminating the interface between the blades and the disc, a braze trap in the form of a small chamber is provided. Experience shows that this chamber becomes filled with braze material during the brazing step.